Cuckoos May Be Dim, But They Are Strategic

April 20, 2005

It's been decades since Bob Payne set out on what some might consider an odd quest. As a graduate student in the 1960s, he wanted to know why some birds that are members of the cuckoo family lay their eggs in nests that belong to other birds, and then let the surrogate mom raise their young.

Do they do it because they are clever enough to outwit the other birds, leaving the chores of raising kids up to others? Or do they do it because they are a little, well, cuckoo?

The attempt to answer that question first took him to Africa, and subsequently on at least 20 trips to areas all over the world where cuckoos are found. In this country, he has probed the mating and nesting habits of yellow-billed and black-billed cuckoos, the familiar ani, and that rascal carnivore of the desert southwest, the greater roadrunner.

There's still more cuckoos to be studied, but Payne thinks he has his answer. He spells it all out in a massive new book published by Oxford University Press, "The Cuckoos." Payne, who is the curator of birds and professor of zoology at the University of Michigan, teamed up with Michael D. Sorenson of Boston University to unravel the story of a diverse family of birds that has fascinated cultures from early Native Americans to lovers of Shakespeare.

"What we wanted to understand is how this brood parasitism evolved," Payne says. How come some members of the cuckoo family dropped their eggs in the wrong nest, while others raised their own, or still others engaged in communal nesting and thus shared the maternal chores with others?

Letting Others Do the Work

Payne is an expert on bird songs, and Sorenson is a geneticist, so the two set out to construct an evolutionary tree of the entire cuckoo family. Nobody even knew how many species there were, but the researchers identified 141, and to answer their question they tried to determine which species were the most closely related.

There was some thought that the parasites were closely related to the cuckoos that nest in sort of a commune fashion, so it was only a small leap into parasitism.

"But it turned out that the cooperative breeding birds aren't closely related to the parasites," Payne says.

As they constructed their evolutionary tree, the researchers found that parasitic behavior emerged three different times in three different lineages. All three sometimes lay their eggs in someone else's nest.

But why? The answer, it turns out, can be found in the numbers. Parasitic cuckoos lay at least some of their eggs in other birds nests because they get more babies that way, Payne says.

"Darwin said that 150 years ago, and we found out that he was right," Payne says, chuckling.

Apparently, the parasitic cuckoo knows that if you let someone else raise some of your kids, you can devote more energy and resources to laying more eggs and producing a lot more cuckoos than if you do it all yourself.

"I thought maybe they were being pretty clever," Payne says. "Or maybe they're just sort of slow and don't have the brain equipment to do it all themselves."

Small-Brained, But Strategic

To find out, he and a colleague measured the one feature that is generally accepted as an indication of intelligence: Brain size, relative to body size. Alas, when it comes to brains, the parasitic cuckoo is more than a few fries short of supersized.

"It turned out that all three groups of parasites have smaller brains" than their kinfolk, Payne says.